PROJECT SUMMARY/ABSTRACT Regulation of chromosome structure is fundamental for genome function. Across eukaryotes, a key regulator of chromosome structure is an evolutionarily conserved protein complex called condensin, which is essential for chromosome condensation and segregation during cell division and play key roles in gene regulation during interphase. The molecular mechanisms behind how condensins bind and regulate chromosome structure and how this affects transcription remain unclear. To address this, we use a specialized condensin that functions within the X chromosome dosage compensation complex (DCC) in C. elegans. DCC specifically binds to and represses transcription of both X chromosomes in hermaphrodites by a factor of two. The co-option of condensin for X chromosome dosage compensation provides a powerful experimental model to study the mechanisms that control specificity of condensin binding and to analyze condensin-mediated changes in chromosome structure and transcription with high precision, all free from potential indirect effects on chromosome segregation. Our previous work suggests that specific and robust DCC binding to the X chromosomes is accomplished by a step-wise recruitment mechanism followed by linear spreading. First, DCC enters the chromosome at a small number of X-specific sites defined by two genomic features: the presence of multiple 12-bp recruitment motifs and overlap with high occupancy transcription factor target sites. After X-specific entry, additional sites cooperate over long-distance to increase the level of DCC recruitment across the chromosome. From the recruitment sites, DCC spreads linearly along large chromosomal domains, accumulating at active gene regulatory elements across the X. DCC binding leads to chromosome-wide transcriptional repression, changes in the level of specific histone modifications, chromosome compaction, and long-range chromosomal interactions. Here, we will address several important questions regarding DCC recruitment, spreading and function using a powerful set of genetic, genomic and imaging approaches: 1) How does the DCC recognize features of the initial entry sites on the X? 2) What is the mechanism behind long-distance cooperation between DCC recruitment elements? 3) How is DCC spreading and DCC-mediated chromosomal interactions regulated? 4) What is the mechanism by which the DCC represses transcription? The outcome of our work will elucidate the basic molecular mechanisms by which condensins perform their wide-range of essential functions in eukaryotes. This is relevant to human health because condensin structure and function is deeply conserved from C. elegans to humans, and determining how condensins function is key to understanding the contribution of chromosome structure to genome function in health and disease.